Particularly by virtue of their light weight, high strength-to-weight ratio, and good recyclability, magnesium-based alloys are promising candidates for many engineering applications, including automotive applications. Indeed, there is substantial interest in replacing steel and aluminum parts in machinery and transportation vehicles with magnesium alloys. However, the corrosion susceptibility of magnesium alloys remains a particular issue of significant concern. Magnesium alloys are susceptible to corrosion because of their high reactivity and low electrode potential (E0=2.37 V), which prevents them from widespread applications. The corrosion mechanisms of magnesium alloys have been well-studied, and can generally include galvanic corrosion, intergranular corrosion, stress corrosion cracking, and/or corrosion fatigue.
Various types of treatments have been used for protecting magnesium surfaces, such as laser and ion beams, PVD, CVD, chemical conversion, anodization, electro- or electro-less plating, organic coating, and others. Among them, chemical conversion coatings are commonly used as paint bases and in some cases as stand-alone protection against atmospheric tarnishing. Traditional conversion coatings are based on hexavalent chromium compounds, but these are associated with severe environmental risks. Some chromate-free conversion coatings have been studied, such as stannate, cerium, aluminum, zirconium, niobium, zinc phosphate, or phosphate permanganate coatings. However, current conversion coating technologies of the art are generally incapable of providing sufficiently passivated magnesium alloys suitable for use in critical applications, such as machinery and automobiles, where mechanical resilience and high wear-resistance are necessary.